The present invention pertains to a method of manufacturing fluoride-sensitive diaphragms which are employed as ion-sensitive electrodes in electro-analytic measuring practice.
Ion-sensitive electrodes have been widely known in electroanalysis for the determination of the amounts of ions. Various ion-sensitive electrodes have been developed during last years for detecting a great number of various ions.
In addition to known glass electrodes, LaF.sub.3 -monocrystal electrodes have also shown very good sensitivity, high selectivity and stability.
A non-porous diaphragm of difficultly soluble metal fluorides has been disclosed, for example in German Pat. No. 1,598,895.
At the present time, only monocrystal structures of LaF.sub.3 with various dopings are used as fluoride-sensitive diaphragms. Due to high resistance of LaF.sub.3 a doping has been necessary for monocrystal electrodes to improve conductivity in general (with Europium fluoride). The reduction of thickness of the diaphragm to reduce resistance has, however, failed up to now and led to non-satisfactory mechanical stability.
To use a LaF.sub.3 -monocrystal electrode, a monocrystal has been up till now glued into a tube of non-conductive material. An electrolyte solution containing fluoride ions was fed into that tube and an electric contact was provided between the electrolyte and a reference electrode. The utilization of such internal reference solution made the manufacture of electrodes difficult and has been often a cause for defects in the electrodes.
The manufacture of monocrystal electrodes has been very expensive which led to high costs of the electrodes.
Tests have been conducted to overcome deficiencies of LaF.sub.3 -monocrystals. It has been proposed, for example, to embed LaF.sub.3 into a matrix of rubber, as disclosed in A. M. G. MacDonald, K. Toth, Anal. Chem. Acta 41, 99, 1968.
The utlization of LaF.sub.3 -stampings and an electro-chemical production of LaF.sub.3 have been disclosed by G. Uhlmann, Dissertation A, LEUNA, Merseburg, 1981.
All above described proposals were not carried through because sensitivity and long-time stability of monocrystal electrodes were insufficient. Tests for a drain of potential from a LaF.sub.3 monocrystal have been carried out with an Ag/Ag F-contact. (T. A. Fjeldly, K. Nagy, J. Electrochem. Soc. 127, 1299, 1980). The manufacture of the contact by melting in inert gas has been, however, very expensive and this could not overcome high costs of the improved LaF.sub.3 -monocrystal.
Research has been conducted, which showed that, in order to avoid the above disadvantages, poly-silicon-conductive paths connected with the gate of the field-effect transistor have been coated with LaF.sub.3, whereby the whole structure up to the fluoride-sensitive area has been coated with photo varnish (see J. van der Spiegel, u.a., "Sensors and Actuators", 4, 291, 1983).
The disadvantages of this technical solution are an extremely large potential drift which practically did not permit the use of such electrodes and also insufficient sensitivity in an ion analysis.